MLA

Voynich, Wilfred Michael, 1865-1930. Johannes Marcus Marci of Cronland, autograph letter. Box A. AD 1666 [or 1665?]. https://collections.library.yale.edu/catalog/2041454.

APA, 6th edition

Voynich, Wilfred Michael, 1865-1930. (["Box A"])(AD 1666 [or 1665?])[Johannes Marcus Marci of Cronland, autograph letter]. https://collections.library.yale.edu/catalog/2041454.